It is known from the prior art, for example U.S. Pat. No. 7,897,095 B2, to produce shell elements, which are made of CFRP material and are reinforced with support elements, by first providing a cured skin element and then placing a plurality of support elements, in the form of stringer elements, onto the skin element in a longitudinal direction and parallel to one another, after which prepreg layers are placed on the lateral surfaces of the support elements, on both sides and in the form of angle brackets, and on the inner surface of the skin element. The prepreg layers are cured by means of an externally applied pressure and a certain temperature such that the support elements are fixed in position on the skin element.
A limitation of this approach, however, is that the support elements can be applied on the skin element only in a longitudinal direction, i.e., parallel to one another. In many applications, however, it is necessary for the skin element of the shell element to be stiffened in both surface directions of the skin element, i.e., also in a transverse direction, perpendicular to the support elements extending in the longitudinal direction. In order to provide support elements extending in the transverse direction in addition to the support elements extending in the longitudinal direction, a solution would have to be found in which the support elements extending in the longitudinal and transverse directions can cross over one another without this weakening the structure in the region of the intersection, i.e., without this largely interrupting the transmission of force.
To this end, it is known in the prior art, for example, to provide cuboid or box-shaped spacers, which are arranged on the skin element parallel to one another in a grid shape, and to then arrange support elements between the spacers, which are then connected with prepreg layers and are subsequently cured. After curing, the spacers can be removed and the result is a honeycombed structure comprising intersecting support elements on the inner surface of the skin element.
However, such a method is disadvantageous for the simple reason that it is very tedious and time-consuming to precisely place the spacers on the skin element in a defined manner and to arrange the support elements between these spacers and connect them.
It is therefore an objective to provide a method for manufacturing a shell element that is reinforced with support elements, wherein it is also possible to provide support elements that cross over one another and extend transversely to one another, on the inner surface of the skin element quickly, without a greater amount of effort, and without this weakening the structure or interrupting the transmission of force in one direction.
This object is achieved by a method for manufacturing a shell element reinforced with support elements, the method having the steps described in the following.
First, a skin element having an outer surface and an inner surface, which is opposite the outer surface, is provided (step (a)). In this context, a skin element is intended to mean a flat structure having a substantially greater extension in a plane parallel to the outer and inner surfaces than perpendicular to this plane, such as the outer skin of an aircraft structure, for example. The skin element is preferably formed of CFRP material.
Subsequently, a first support element is provided that extends along a first axis, in particular longitudinally, and has an attaching side parallel to the first axis and a free side opposite the attaching side (step (b)). Such a support element preferably has a substantially greater extension in a direction parallel to the first axis, along the said support element, and in a direction from the attaching side to the free side, than in a thickness direction perpendicular to these two aforementioned directions. Preferably this is a stringer element or a comparable structural element for stiffening the outer skin of an aircraft structure. The first support element is preferably formed of CFRP material.
As the next step, the first support element is attached on the skin element such that the attaching side of the first support element faces the inner surface of the skin element (step (c)). The attaching side of the first support element preferably rests on the inner surface of the skin element, in order to thereby absorb loads from the skin element.
Next, a first recess, preferably in the form of a slot, is formed on the first support element the slot extending away from the free side of the first support element and transversely to the first axis (step (d)). The forming of the first recess and the attachment of the first support element on the skin element can also take place in reverse order, i.e., steps (c) and (d) are interchangeable.
In the subsequent step, a second support element is provided, which extends along a second axis, preferably longitudinally, and has a support side and a front side opposite the support side (step (e)). The second support element is preferably designed in a manner analogous to the first support element, in particular in terms of the geometry thereof. Preferably this is a stringer element or a comparable stiffening element of an aircraft structure. The second support element is preferably made of CFRP material.
Next, the second support element is inserted into the first recess of the first support element such that the support side faces the inner surface of the skin element and such that the second axis of the second support element extends transversely to the first axis of the first support element (step (f)). The width of the first recess preferably corresponds to the width of the second support element, although a slight amount of play is permissible. After the support element is inserted into the first recess, the support side preferably rests on the inner surface of the skin element, thereby enabling the second support element to absorb loads from the skin element. The arrangement of the first support element and the second support element transversely to one another or such that the first axis and the second axis extend substantially transversely to one another is intended to mean that deviations from a 90° angle between the two axes should be included and that “transversely” is therefore intended to be broadly interpreted.
Finally, the second support element is attached on the first support element and/or on the skin element (step (g)). The second support element can therefore be attached on the first support element and, additionally or alternatively, on the skin element.
A method comprising the aforementioned steps therefore makes it possible to apply support elements that cross over one another and extend transversely to one another, on the inner surface of a skin element in a relatively simple and rapid manner, wherein an interruption of force does not occur in either of the two directions of extension of the support elements, and the shell element can therefore be designed to be equally, or nearly equally, stiff and durable in both directions.
In one preferred embodiment, a second recess, preferably in the form of a slot, is formed on the second support element, the slot extending away from the support side of the second support element and transversely to the second axis. In this case, the second support element is inserted into the first recess of the first support element at the location of the second recess while step (f) is being carried out, and therefore the first recess and the second recess intersect during the insertion, i.e., these engage into one another, and the sections of the second support element located next to the second recess encompass the first support element. A first end point of the first recess is then opposite a second end point of the second recess and, preferably, can also rest thereon. The width of the second recess preferably substantially corresponds to the width of the first support element, wherein a certain amount of play is permissible. A recess is thereby provided not only on the first support element, but also on the second support element, thereby making it possible to coordinate the extensions of the two recesses with one another in order to best compensate for the weakening of the structure associated with the recess.
In this regard, it is preferable for the first recess to extend away from the free side to a first end point that is spaced apart from the attaching side by a first distance, for the second recess to extend away from the support side to a second end point that is spaced apart from the front side by a second distance, and for the length of the first distance to correspond to the length of the second recess from the support side to the second end point. However, the length of the first distance can also be slightly shorter or longer than the length of the second recess from the support side to the second end point, and does not need to be identical thereto. As a result, the support side of the second support element can lie in a plane with the attaching side of the first support element, wherein both of these can preferably rest on the inner surface of the skin element.
The length of the second distance can preferably correspond to the length of the first recess from the free side to the first end point, thereby enabling the front side of the second support element to lock with the free side of the first support element. However, this can also be longer or shorter than the length of the first recess from the free side to the first end point such that one of the two support elements extends above the other. The front side and the free side of the support elements can then be brought to one level, preferably by means of material removal, for example by grinding or milling.
In a further embodiment, a projection is provided on the first support element and extends away from the skin element on the free side, wherein the first recess is formed in the projection. Since the recess is formed in the projection, this recess therefore extends less deeply into the first support element, and therefore the structure of the first support element is weakened to a lesser extent.
Preferably, the projection is formed by removing material from the first support element, on the free side thereof, on both sides of the first recess and at a distance therefrom. That is, the material of the first support element is removed on the free side thereof, in the direction toward the inner surface of the skin element, preferably along the entire extension of the support element along the first axis, except for a region adjoining the recess on both sides where material is not removed, the region therefore forming the projection. The material removal can be implemented by means of grinding or milling, for example. This step of material removal can be carried out after the first support element is attached on the skin element (step (c)) or before this step. In addition, the first recess can be introduced before or after the material is removed.
Furthermore, the first end point of the first recess lies in the plane in which the free side extends next to the projection. It is not necessary for the end point to be located exactly in this plane; instead, the end point can be located slightly above or below this plane. As a result, the first recess is formed entirely in the projection and therefore does not result in a weakening of the structure of the first support element.
According to a further embodiment, the first support element is attached on the skin element by means of prepreg layers, which are preferably made of CFRP material and which are placed onto the lateral surfaces of the first support element, between the attaching side and the free side, and on the inner surface of the skin element, next to the first support element, and which are subsequently cured. At least one prepreg layer is used in this case, although a plurality of prepreg layers lying one above the other is preferably used. The prepreg layers are placed onto at least one of the two lateral surfaces of the first support element and onto the adjacent region of the inner surface of the skin element, although the prepreg layers are preferably placed onto the two opposing lateral surfaces of the first support element and onto the corresponding adjacent regions of the inner surface of the skin element. The prepreg layers are placed onto the described surfaces in the non-cured, flexible state and are subsequently cured by means of a certain externally applied pressure and an appropriate externally applied temperature, whereby these prepreg layers stiffen and are fixed in the shape thereof in which they were applied.
Such a process of attaching a first support element on a skin element by means of prepreg layers is extensively described in U.S. Pat. No. 7,897,095 B2, the disclosure of which is hereby included in the present application.
According to a further embodiment, the second support element is attached on the first support element by means of prepreg layers, which are preferably made of CFRP material and which are placed onto the lateral surfaces of the first support element, between the attaching side and the free side, and onto the inner surfaces of the second support element, between the support side and the front side, and which are subsequently cured. In a manner analogous to the attachment of the first support element on the skin element by means of prepreg layers, it is also possible to provide one or a plurality of prepreg layers for attaching the second support element on the first support element. The prepreg layers are provided on one or both opposing sides or in all four corners between the first support element and the second support element.
In yet another embodiment, which can be carried out as an alternative to or in addition to the aforementioned embodiment, the second support element is attached on the skin element by means of prepreg layers that are preferably made of CFRP material that and are placed onto the lateral surfaces of the second support element, between the support side and the front side, and onto the inner surface of the skin element, next to the second support element, and which are subsequently cured. In this embodiment, which is analogous to the above-described embodiments, at least one and preferably a plurality of prepreg layers are provided on at least one lateral surface, preferably on both lateral surfaces, of the second support element and the corresponding adjacent regions of the inner surface of the skin element.
The attachment of the first support element and the second support element to one another and/or to the skin element by means of prepreg layers is particularly advantageous since the prepreg layers, in the uncured state, can be applied very flexibly into the corners between the support elements and the skin element and, in the cured state, impart great stiffness and strength, combined with low weight.
According to yet another embodiment, after the second support element has been attached on the first support element and/or on the skin element (step (g)), the projection is removed to a level in which the free side extends next to the projection. This can be carried out by means of milling or grinding, for example. In this manner, a uniform, level surface is created on the free side of the first support element, which, preferably, can also extend in a plane with the front side of the second support element. The projection was used in this case merely to hold the second support element on the first support element until these were attached by means of the prepreg layers, whereupon the projection can be removed in its entirety, i.e., after the prepreg layers have cured and the two support elements are fixed to one another and to the skin element.
In one embodiment, further support elements are provided on the skin element parallel to the first and/or second support element in a manner that is identical or similar to the first and/or second support element. In this manner, a grid comprising a plurality of support elements extending in parallel and transversely, i.e., crossing over one another, can be formed, the support elements supporting the skin element on the inner surface thereof and therefore forming a reinforced shell element. The prepreg layers can be applied continuously over a plurality of parallel support elements. The process of attaching the support elements on the skin element by means of prepreg layers can be simplified and accelerated in this manner.